Wallboard is generally installed in large panels, which are nailed, screwed, or glued to the studding of walls of buildings. The joints where sections of the wallboard are butted together are covered with a joint compound and then a fiberglass or paper reinforcing tape is embedded within the joint compound and then permitted to dry. When the joint compound is dry, a second application of the joint compound is applied over the joint and is permitted to dry. A coating of the joint compound is also applied to cover nail heads or screws or any cracks in the wallboard and let dry. After the joint compound dries, the joint and covering of the nails or screws are lightly sanded and the wall is then finished with decorating material such as paint.
Cellulose ethers (CEs) represent an important class of commercially important water-soluble polymers for use as rheology modifiers. These CEs are capable of increasing viscosity of aqueous media. This viscosifying ability of a CE is primarily controlled by the CE's molecular weight, chemical substituents reacted with the cellulose, and conformational characteristics of the polymer chain. CEs are used in a broad range of different application fields and products such as construction, paints, and a wide variety of other applications.
Cellulose ethers used in tape joint compounds must function to increase the viscosity of the joint compound, and provide sufficient water retention, allowing the troweled joint compound to wet the wallboard and tape substrates at a controlled rate so that penetration of the compound into the substrates occurs. Upon drying, a strong adhesion bond among the joint compound, wallboard and paper tape is then achieved. The cellulose ether also controls the joint compound rheological properties, making it easier for the craftsman to apply and trowel the compound to form a smooth, homogeneous surface on the substrate. The compound must retain its water for extended periods of time, so that the artisan may work with a given aliquot of joint compound that does not dry out quickly due to rapid water loss by either evaporation or absorption of the water into the substrates. To achieve these properties, methylhydroxypropylcellulose (MHPC), methylhydroxyethylcellulose (MHEC), hydroxyethylcellulose (HEC), hydrophobically modified hydroxyethylcellulose HMHEC), and blends thereof are typically used. It is known that the higher the molecular weight of the cellulose ether, the stronger is the immobilization of the water in joint compounds and other construction materials.
A second way to benefit from high water demand is to leave the water level unchanged, but lower or eliminate the clay. Ready mix joint compounds contain clay as an agent to impart needed rheology and other properties to the compound. Controlled interaction with the water-soluble polymer (WSP) is needed to maximize these properties. Even when these interactions are under control, there are often other difficulties when the clay is present at typical (1.5–3 wt %) levels; these difficulties include: 1) cracking of the joint compound upon drying; 2) thickening of the joint compound as it ages; and 3) the need to normalize clay and water levels due to the variable nature of clay, a natural product. These are the most common difficulties with clay, especially attapulgite which is typically used.
A need still exists in the joint compound industry for a water retention agent that can be used in a cost effective manner to improve the application and performance properties of joint compounds. In order to assist in achieving this result, it would be preferred to provide a water retention agent that provides a 2% aqueous solution Brookfield viscosity preferably greater than about 80,000 mPas (as measured using a Brookfield RVT viscometer at 20° C. and at 20 rpm using Spindle number 7) and still be cost effective for use as a thickener and water retention agent. There is also a need to eliminate the above mentioned difficulties associated with the use of clay in joint compound formulations. Therefore, it is desirable to eliminate or reduce the use level of clay injoint compound formulations.
U.S. patent application Ser. No. 10/939,815, filed Sep. 13, 2004, discloses the use of specific carboxymethylcellulose (CMC) derivatives in order to significantly reduce the amount of clay in a joint compound. While this technology is solid and promising, situations could arise in which it may be difficult to employ CMC in a joint compound. For example, when dolomitic limestone or inorganic salts are present in the formulation, the CMC, which is an anionic polymer, may interact with limestone or inorganic salts and may lose some of its effectiveness as a thickener and water retention agent. By using a nonionic water soluble polymer (WSP), such a possibility could be precluded.